The present invention relates generally to linerless prepregs and composite articles made therefrom, as well as methods of making and using the same.
“Composite materials” and “composite articles” made therefrom are based on at least one reinforced matrix material, for example, a fiber-reinforced polymeric resin. The combination of the matrix material (e.g., resin) and the reinforcing material (e.g., fibers) often produces extremely strong articles that are also lightweight. Thus, composite materials are finding increased use in applications where lightweight materials are desired and where an associated compromise in strength or stiffness of the material would likely be problematic. Many composite materials are also useful in applications where corrosion resistance is desired, as composite materials more often exhibit excellent corrosion resistance as compared to alternative materials.
Due to their beneficial properties, a variety of specialized sporting implements and other articles are increasingly being made from composite materials. For example, composite materials are increasingly being used in manufacturing shaft-based sporting implements (i.e., those sporting implements having a generally elongated portion, which may or may not be hollow or uniform in thickness and shape throughout) and similar articles. Such articles include, for example, golf clubs, bicycle frames, hockey sticks, lacrosse sticks, skis, ski poles, fishing rods, tennis rackets, arrows, polo mallets, and bats. As an example, the use of composite materials enables golf club manufacturers to produce shafts having varying degrees of strength, flexibility and torsional stiffness.
In addition, a variety of articles in the transportation and energy industries are increasingly being made from composite materials. For example, composite materials are often used to make various aerospace components, such as wing and blade components, including those on helicopters and specialized military aircraft. Further, composite materials are often used to make various automotive components, both interior and exterior, including body panels, roofs, doors, gear shift knobs, seat frames, steering wheels, and others. In the energy industry, composite materials are used to make wind mill blades—e.g., large wind turbine blades are made more efficient through the use of carbon fiber-reinforced composites. Indeed, the number of current and potential applications for composite materials is extensive.
One method of manufacturing composite articles is termed a “wet” process, which involves the use of prepreg. “Prepreg” refers to pre-impregnated composite reinforcement material, where the prepreg contains an amount of matrix material used to bond the reinforcement material together and to other components during manufacture.
Exemplary matrix materials include epoxy resins, phenolic resins, bismaleimide resins, polyimide resins and other thermosetting resins. Epoxy resins tend to be the most common. While the prepreg can contain any suitable reinforcement material, fibrous reinforcement is common. Such reinforcement can be continuous or discrete. Exemplary forms of continuous fiber reinforcement include those containing woven, mat, random, or uni-directional fibers. Typical fibrous reinforcement materials include carbon fibers, glass fibers, aramid fibers, boron fibers, polyethylene fibers, and others.
During manufacture of composite articles therefrom, prepreg is typically provided in roll form, where a sheet of prepreg can be cut from the roll. In many instances, the matrix material is a thermosettable resin that has not been fully cured. Thus, in composite article manufacturing, prepreg containing uncured thermosettable resin is often layered and/or wrapped around a preform or mold. Then, heat and/or pressure is applied to cure (i.e., crosslink) the thermosettable resin of the prepreg. Once the matrix material is cured, the composite article has essentially the desired shape, and the reinforcement material is locked into position by the cured matrix material.
Generally, prepreg is manufactured separately from the composite articles made therefrom. When prepreg is provided to a composite manufacturer, it is typically assembled with one or more release liners, which must be removed prior to manufacturing composite articles therefrom. Typically, release liners are paper-based or polymeric film-based materials (e.g., polyethylene) containing a low surface energy coating (e.g., silicone-based coating) on at least one side thereof to provide release properties.
Release liners prevent the prepreg from sticking to itself when, for example, stacked in sheet form or rolled onto a core. In some instances, especially when the prepreg is supplied in roll form, sticking may even prevent the prepreg from being unwound without destroying or significantly distorting the reinforcement material within the prepreg.
Prepreg is typically manufactured using one of two different methodologies: (1) solution method, and (2) film transfer method. During one embodiment of the solution method of manufacturing, the reinforcement material is passed through a bath of uncured matrix material in solution form. The coated reinforcement material is then passed through an oven to partially cure the matrix material (often known as B-staging the material) and/or to remove any solvents that may have been used in coating the matrix material. After exiting the oven as a prepreg, a release liner is laminated to at least one side of the coated reinforcement material. The prepreg assembly can then be easily wound onto a core for later use.
In an alternative embodiment of the solution method of manufacturing, the uncured matrix material is coated directly onto a release liner. The reinforcement material is then positioned within the uncured matrix material coating, after which the assembly is passed through an oven to partially cure the matrix material and/or remove solvents. After exiting the oven as a prepreg, a second release liner is laminated to the opposite side of the assembly from that having the first release liner. The prepreg assembly can then be easily wound onto a core for later use.
In either version of the solution manufacturing method, all steps can be and are desirably performed as part of a continuous process. For example, FIG. 1 schematically illustrates one variation of a continuous solution method for manufacture of prepreg, where the reinforcement material is passed through a bath of uncured matrix material. As illustrated therein, a reinforcement material 110 is unwound and passed through a bath of matrix material 112 before passing through nip rollers 114. Thereafter, the coated reinforcement material 116 is passed by various heating elements 118 within an oven 120. After exiting the oven 120, release liners 122 and 124 are assembled on opposite sides of the partially cured assembly 126 (i.e., a partially cured matrix material containing a reinforcement material) to form a prepreg assembly 128. The prepreg assembly is then wound onto a core 130 for storage and later use.
As another example, FIG. 2 schematically illustrates another variation of a continuous solution method for manufacture of prepreg where the uncured resin material is coated directly onto a release liner. As illustrated therein, a first release liner 210 is unwound and passed adjacent a matrix material reservoir 212, which dispenses matrix material thereon. Thereafter, a reinforcement material 214 is laminated to the matrix material of the coated assembly 216 by passing the entire assembly 218 through an oven 220 after contacting the reinforcement material 214 therewith. After exiting the oven 220, a second release liner 222 is added to the laminated prepreg assembly 224 on the opposite side as that containing the first release liner 210. The prepreg 226 is passed through nip rollers 238 and then wound onto a core 230 for storage and later use.
According to the film transfer method, matrix material is coated onto a first release liner. A second release liner is then laminated onto the opposite side of the matrix material to form a matrix film. Then, during a separate manufacturing step, one of the first and second release liners is removed from the assembly to expose the matrix material. Next, reinforcement material is laminated to the matrix material using heat and pressure. The heat of this lamination step acts to partially cure the matrix material. Finally, a third release liner is laminated to the resulting prepreg assembly opposite from the other release liner. The prepreg assembly can then be wound onto a core for later use.
The film transfer manufacturing method can occur in two separate continuous steps. For example, FIGS. 3A and 3B illustrate two such steps in a film transfer method for manufacture of prepreg. The first continuous step, a film production step, is illustrated in FIG. 3A. A first release liner 310 is unwound and passed under a matrix material reservoir 312, which dispenses matrix material onto the release liner 310 through a coating head 314. Thereafter, a second release liner 316 is unwound and laminated onto the assembly on the side opposite from the first release liner 310. The resulting film of matrix material 318 is then wound onto a roll 320 for use in the second step.
The second continuous step, a film transfer step, is illustrated in FIG. 3B. The film of matrix material 318 is unwound and contacted on one side, where one of the release liners 310 and 316 has been removed, with a reinforcement material 322 that is then contacted with another film of matrix material 318 for lamination using a source of heat 324 and a source of pressure 326. After lamination, the second film of matrix material 318 added to the assembly is removed and replaced by another release liner 328, after which the assembly is passed through nip rollers 330. The resulting prepreg assembly 332 is then wound onto a core 334 for storage and later use.
Many conventional prepregs are made using solvent-based methods (e.g., solution methods). When using solvent-based methods, viscosity of the resin material is decreased by the addition of sufficient amounts of solvent to enable adequate impregnation of the reinforcement material. The solvent is typically removed subsequently via heating. While this method facilitates impregnation of the reinforcement material without the use of heat, the use of solvents makes the process more expensive, less environmentally friendly, and potentially hazardous. Furthermore, the use of a solvent-based method can result in the undesirable retention of solvent within the final prepreg. During subsequent cure of the prepreg, residual solvent is prone to volatilization (e.g., upon heating). Volatilization of the solvent can create unwanted voids within the resulting composite article. Additionally, the need for heating may also preclude certain curing agents from being utilized in the matrix material, thus limiting design flexibility.
Hot-melt processing is an alternative method that has been used when forming conventional prepregs. “Hot-melt processing” refers to processing of essentially 100% solid systems. “Hot-melt processable” refers to those systems that can be, but are not required to be, processed using hot-melt processing techniques. Usually, hot-melt processable systems have no more than about 5% organic solvents or water, more typically no more than about 3% organic solvents or water. Most typically, such systems are free of organic solvents and water. Not surprisingly, methods of this type are often preferred over solvent-based methods. However, because conventional prepregs are often based on matrix materials having a relatively high viscosity, heat in excess of 60° C. is often needed to sufficiently reduce the viscosity so that the matrix material can adequately infiltrate the reinforcement material during formation of the prepreg using hot-melt processing techniques. When the matrix material includes a curative that is heat-activated, hot-melt processing may result in premature cure thereof if not handled appropriately. Thus, the type of curative used can make it impractical to heat the matrix material to the temperature necessary for imparting the desired processing viscosity.
In addition to issues associated with solvent-based and hot-melt processing methods, there are several disadvantages associated with manufacturing and using a prepreg with one or more release liners. Generally, any release liners present are ultimately removed and discarded during subsequent composite article manufacturing. As such, the presence of the release liners serves no functional purpose in the final composite article and only adds cost to the prepreg. Further, the release liners are generally removed from the prepreg manually during composite article manufacturing. This can often be a time-consuming process, especially since gloves usually need to be worn by those handling prepregs. In addition, release liners often contain a silicone-based release coating or other coating containing release agents, which can lead to contamination in the final composite article and assemblies thereof. Further, such contamination can cause layers of prepreg-derived reinforcement to delaminate in final composite articles.
By far, the potential for silicone contamination (or contamination from other release agents) is the most alarming issue associated with using prepregs having one or more release liners assembled therewith. For example, to create a silicone-based release coating for a release liner, a silicone-based material is typically coated and then cured (i.e., crosslinked) onto a substrate (e.g., polymeric film or paper). Notwithstanding the best manufacturing techniques, there is always the potential for uncured silicone contaminants to remain in the resulting release liner. Silicone contaminants can be transferred to the uncured matrix material within a prepreg and, ultimately, into the final composite article and further assemblies made therefrom. The presence of silicone or other types of release agent contaminants can cause reinforcement layers made from the prepreg within the composite article to delaminate, possibly rendering the composite article useless. Delamination can have particularly catastrophic consequences when the article is used in critical structural applications (e.g., in the aerospace industry).
Even when release agent contamination is not a concern, the use of a release liner is generally undesirable. As discussed, the use of a release liner decreases process efficiency when preparing composite articles based on prepregs assembled with one or more release liners that need to be removed during manufacture of the composite article. Efficiency is reduced both in terms of time and cost, which can vary considerably depending on chemistry of the release liner. For example, polymeric release liners are often made from fluoro-polymers due to their inherently good release properties. However, the price of such fluoropolymer release liners is often about five to ten times the cost of traditional silicone-based release liners.
Thus, for many reasons, there is a need to eliminate release liners from prepreg assemblies used in manufacturing composite articles. It is also desirable to provide alternative methods for preparation of prepregs as compared to conventional solvent-based and hot-melt processes.